1. Field of the Invention
This invention relates, in general, to a balanced mixer, and, more particularly, to a balanced mixer for use in RF frequencies.
2. Description of the Background
Currently in a typical microwave/millimeter wave down converter the detection bandwidth determining receiver sensitivity is set by filtering at the first IF amplifier input. The filter acts as a resistive match at the lower sideband and is reactive at the upper sideband. As a result the upper sideband is reflected back into the IF port of the mixer where it is combined again with the local oscillator (LO) frequency of the mixer to produce a signal having a 2F.sub.LO +F.sub.RF DC term and a F.sub.LO +F.sub.RF -F.sub.LO AC term (the original RF frequency). This effects the conversion loss and the RF port VSWR, depending on the phase of the reflective signal. The conversion loss may be as great as 6 dB. The DC term further degrades the two-tone intermodulation performance by as much as 20 dB. For down converters operating below 3 to 5 GHz, the resistive match to both sidebands at the mixer output can be separatly achieved utilizing complementary filters at the upper and lower sideband frequencies. For down converters operating above this range where the frequency spread between the lower and upper sidebands is rapidly increasing this approach does not work effectively. The upper sideband frequency that is reflected back into the mixer can create an additional mismatch and two-tone intermodulation results in a degradation of the RF signal and a corresponding corruption of the IF signal.